1. Field of the Invention
This invention relates to bicomponent melt-bondable fibers, more particularly, such fibers suitable for use in nonwoven webs.
2. Discussion of the Prior Art
Nonwoven webs comprising melt-bondable fibers and articles made therefrom are an important segment in the nonwovens industry. These melt-bondable fibers allow fabrication of bonded nonwoven articles without the need for the coating and curing of additional adhesives, thereby resulting in economical processes, and, in some cases, fabrication of articles not capable of being made in a conventional manner.
There are two major classes of melt-bondable fibers--unicomponent fibers and bicomponent fibers. A bicomponent melt-bondable fiber is one comprising both a polymer having a high melting point and a polymer having a low melting point. Bicomponent fibers are preferred over unicomponent fibers for several reasons: (1) bicomponent fibers retain their fibrous character even when the low-melting component is at or near its melting temperature, as the high-melting component provides a supporting structure to retain the low-melting component in the general area in which it was applied; (2) the high-melting component provides the bicomponent fibers with additional strength; (3) bicomponent fibers provide loftier, more open webs than do unicomponent fibers. Bicomponent fibers are known to suffer from the following problems:
(1) Excessive thermal shrinkage. Bicomponent fibers have great latent crimp, resulting from thermal shrinkage occurring at the same time as crimp generation. In web bonding, high shrinkage results in nonwovens uneven in density and lacking in uniformity of width and thickness. PA0 (2) Splitting of component elements. Polymers arranged either side-by-side or as sheath core fibers are easily detached in the fiber state or in the nonwoven manufacturing process. PA0 (3) Difficulty in spinning fine fibers. It is very difficult to obtain melt-bondable bicomponent fibers finer than six denier. PA0 (a) forming a plurality of unstretched side-by-side composite fibers consisting of a first component comprised mainly of crystalline polypropylene and a second component composed mainly of at least one olefin polymer other than crystalline polypropylene, PA0 (b) stretching said unstretched composite fibers at a stretching temperature at or above 20.degree. C. below the melting point of said second component, PA0 (c) incorporating said stretched fibers having 12 crimps or less per 23 mm into a web, PA0 (d) subjecting said web to heat treatment at a temperature higher than the melting point of said second component but lower than the melting point of said polypropylene whereby said nonwoven fabric is stabilized mainly by melt adhesion of said second component of said composite fibers.
Shrinkage of the web per se is not necessarily a problem. However, shrinkage is accompanied by severe curling and agglomerating of individual fibers, particularly at the points where they join. Buffing pads made of nonwoven fibers must be sufficiently uniform so that they do not mar the smooth finish of a floor when used thereon. Because of the aforementioned curling and agglomerating of the fibers in the pad, fine abrasive particles that are typically added to the pad tend to become concentrated at the points where the fibers agglomerate, i.e. the junction points thereof. This nonuniformity of abrasive distribution generally results in marring of floors during the cleaning and buffing thereof.
Kranz et al, U.S. Pat. No. 3,589,956 discloses a product made by a process wherein sheath-core bicomponent continuous strands are mechanically crimped and annealed into form, then cut to staple length and formed into a nonwoven assembly, then heated and cooled to bond. Drawing treatments performed subsequent to the spinning operation create internal stresses within the filaments and these tend to result in undesirably high shrinkage and/or crimping forces should the filaments be heated above their second-order transition temperature, i.e. of the filamentary component. Accordingly, the filaments are stabilized, e.g. by annealing, to relieve these tendencies and thus lower the retractive coefficient.
Tomioka, in an article entitled "Thermobonding Fibers for Nonwovens", Nonwovens Industry, May 1981, pp. 22-31, describes ES bicomponent fiber, which comprises polyethylene and polypropylene in a so-called modified "side-by-side" arrangement. This fiber is also disclosed in Ejima et al, U.S. Pat. No. 4,189,338. The fiber of the Ejima et al patent is prepared by
While heat stabilizing has been shown to be effective in reducing shrinkage of bicomponent fibers, many desirable polymeric materials are not sufficiently resistant to heat to be able to successfully undergo heat stabilization processes. Accordingly, there is a great need to provide bicomponent fibers that do not require heat stabilization in order to minimize shrinkage.